feature: added the pool_size argument to the balancer keepalive FFI API.

The balancer keepalive cache has a single capacity per upstream {} block,
set by balancer_keepalive (32 by default), and evicts the least recently
used idle connection regardless of which pool it belongs to. When many
pools share one upstream, as is typical when peers are picked dynamically
in balancer_by_lua*, a burst to one peer can push out the idle connections
of every other peer.

ngx_http_lua_ffi_balancer_enable_keepalive() now takes a pool_size
argument. A pool is the same key the cache already matches on for reuse:
the peer address, the host given to set_current_peer() and the local
address. When a connection is about to be cached and its pool already
holds pool_size idle connections, the least recently used idle connection
of that same pool is closed and its slot reused, instead of taking a slot
from the global LRU. pool_size 0 keeps the previous behaviour, and
pool_size never raises the balancer_keepalive total.

To keep this O(1), the hash buckets now hold one descriptor per pool
instead of the cached connections themselves. A descriptor keeps the
pool's idle connections (most recently used first) and their count, so
reusing a connection, checking whether a pool is full and evicting its
oldest connection no longer scan the other cached connections. Scanning
the bucket costs about 10ns per idle connection of the pool, i.e. ~5us
per released connection with a full pool of 512. Descriptors are
preallocated next to the cache items, one per item, as a pool only exists
while it holds idle connections.

The key comparison used by the reuse path is factored out into
ngx_http_lua_balancer_item_match().

This changes the FFI signature, lua-resty-core needs to be updated
accordingly.

Author: oowl